Fissionable fuel materials such as oxides of uranium, plutonium or thorium, and combinations thereof, are typically formed into small cylindrical pellets and housed within sealed tubes or elongated containers sometimes referred to in the art as "cladding". The sealed fuel containers protect the fuel from reacting with the coolant, or any foreign matter entrained therein, and prevent the escape of any fission products from the fuel, which are normally highly radioactive and corrosive, into the coolant and in turn contamination of the overall system. Thus the enduring integrity of the sealed container housing the fissionable fuel is therefore of the utmost importance.
Large capacity power generating nuclear fission reactor plants normally employ several hundreds of such sealed tubular containers housing fissionable fuel. To facilitate the periodic refueling, which commonly is performed by replacing fractional portions of the total fuel at intervals and rearranging other fractional portions, the tubular fuel containers, or fuel elements consisting of same, are conventional assembled into bundles or groups of approximately 40 to 90 elements which can be handled and manipulated as a single composite unit.
Elongated or tubular containers housing fissionable nuclear fuel materials are therefore assembled into a designed array conventionally comprising a group of spaced apart, parallel aligned tubular containers of fuel secured by mechanical means. A typical fuel bundle comprises, for example, an eight by eight or nine by nine array of spaced fuel containers. The tubular fuel containers usually are several feet in length, such as about 14 feet, and approximately one-half inch in diameter and are each spaced from the others a fraction of an inch. The spacing is required to permit an ample flow of heat removing coolant, such as water, over the full exterior surface of all tubular fuel containers for effective heat transfer and thus effective operation.
To inhibit such elongated fuel elements from bowing and vibrating due to high heat and velocities of coolant flowing thereabout, whereby they can contact each other and in any case impede or unbalance coolant flow, it is necessary to retain the tubular fuel elements in their spaced apart array or relation by means of a plurality of spacing units positioned at intervals along their length.
Typical spacing units for tubular fuel elements comprise a frame having a multiplicity of crossing components or lattice which form or provide a plurality of opening arranged in the designated pattern for spacing the parallel aligned fuel elements. A group of the tubular fuel elements are each inserted into and passed through the aligned opening of a series such spacing units positioned at the intervals along the length of the elements in a predetermined pattern or distance. Thus with each elongated fuel element of a group traversing several spacing units at intervals which provide intermediate restraint and support transverse of the group, the spaced apart, parallel aligned fuel elements are each restrained from lateral bowing and vibration which could damage their structure or impede effective coolant flow intermediate and around each fuel container. A common commercial embodiment of a nuclear reactor fuel bundle has about seven such spacer units securing all tubular fuel elements extending therethrough which are positioned at intervals along the length of the grouped array of elements.
Spacing units for securing bundles of fuel rod elements frequently contain spring and stop members which press against the fuel rod elements in metal to metal contact as a means of securely gripping and holding the fuel rod elements in position.
The assembled bundle of a group of spaced apart, parallel aligned array of the tabular fuel elements secured to each other by traversing through the openings of a series of spacing units positioned at intervals along their length additionally have each of their ends supported in sockets of tie plates. This bundle assembly is also typically surrounded by an open ended tubular channel of suitable cross-section such as square, to direct the flow of coolant longitudinally along the surface of the fuel elements and guide the neutron absorbing fission control rod units which move reciprocally longitudinally intermediate the channel surrounded bundle of fuel elements.
Typical fuel bundle assembles of the foregoing common construction are disclosed in Letters U.S. Pat. No. 3,350,275, issued October 1967 and U.S. Pat. No. 3,654,077, issued Apr. 4, 1972. The disclosed contents of said patents, and those cited therein, are incorporated herein by reference.
Structural components utilized within the reactor core of fissionable fuel, such as the tubular containers housing the fuel and their spacing units, etc. must be fabricated from a durable metal which has a low neutron absorbing capacity, or cross section, so as not to impede the neutron incited fission chain reaction. The preferred material most commonly used comprises alloys of zirconium which have a neutron absorption capacity in the order of about one-fifteenth of that of stainless steel. However zirconium alloys are under certain circumstances susceptible to corrosion which can result in its structural failure. To impede a destructive form of self-perpetuating corrosion peculiar to zirconium and its alloys referred to in the art as modular corrosion, components produced from zirconium alloys, such as fuel containers, are commonly treated to form a specific oxide surface layer which resists modular corrosion and surface attack under reactor conditions.